The thyroid hormone receptor from rat liver nuclei was covalently labeled with the N-bromoacetyl derivatives of L-thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). SDS gel electrophoresis of the labeled nuclear receptor showed one major radioactive component with a molecular weight of 56,000. We have been studying qualitative and quantitative changes of rat liver nuclear proteins in control rats thyroidectomized and thyroidetomized after triiodothyronine (T3) injection. The most profound change was found in a protein of molecular weight 32,000 D. This protein has the highest relative intensity in control, lowest in hypothyroid rats and its intensity gradually increases after 6, 12, 18 and 24 hours after T3 injection, when it reaches 50 percent of the control level. Because of our interest in an effect of thyroid hormones on chemical changes of nuclear proteins, we studied the acetylation of the crude preparation of HMG proteins in control and hypothyroid rats. Three major peaks in a range of molecular weight 18-12,000 D were highly acetylated. At the present time, they have not been conclusively identified, but they co-migrate with the mixture of calf thymus histones.